Ocean-going ships use bunker fuel rather than diesel, but river craft might use diesel. Bunker fuel is very dirty and has to undergo on-board processing in order to make it sufficiently liquid to ignite.
Yes, you are correct, bunker oil or similar heavier grades of oil fuel have a similar carbon intensity to diesel fuel, a bit worse in fact.
So my statement that ships almost always use less fuel than trains, but on average have worse carbon emissions than an electric train still stands.
Steam ships are now largely obsolete. Modern ships use large low speed diesels. Generally run on ordinary diesel fuel in port or when first started. Changed over to heavy oil when running. The heavy oil is strained, heated and then fine filtered before use.